Abstract

Combined analysis of ocean-bottom seismometer (OBS) and streamer data is critical in reducing
uncertainty in sub-basalt exploration. A major concern is that errors in basalt thickness and sub-
basalt velocities could result in a missed target in drilling; a mis-identification of base basalt can
change the geological model. Standard streamer data analysis offers poor velocity estimation below
top basalt while standard OBS data analysis gives weak depth control on base basalt. We provide
extra constraint by interpreting the full offset extent of base basalt reflections and basalt diving
waves. Our multi-step methods are: 1) identifying base basalt wide-angle reflections in OBS data
and then their equivalent in the streamer data; 2) tracing the near-offset continuation of both wide-
angle reflections into the respective water wave cones by means of multiple removal through
predictive deconvolution, block shift and fk-filtering; 3) inverse ray modelling the basalt velocity
using refracted basalt arrivals and base basalt depths from the full offset range of the base basalt
reflection; 4) inputting model results to pre-stack depth migration velocity for modelling deeper
layers. Here, we present results using data from the integrated Seismic Imaging and Modelling of
Margins (iSIMM) Project, acquired across the Faroe-Shetland basin. Our results show well-
constrained basalt velocities increasing from 4.5km/s at the top to 5.5km/s at the base, and
thicknesses of up to 3km where basalts overlie a low velocity zone. A geologically-satisfying model
of this low velocity zone is presented, with its base matching an observed basement reflection.